In a hybrid vehicle equipped with an internal combustion engine, an electric motor, an electric generator and a battery, the operating condition of the internal combustion engine is controlled so as to follow an operation line defined in an operation map using the rotational speed and the torque of the engine as target parameters. JP2009-215898A discloses such a hybrid vehicle in which a plurality of operation lines are provided. An operation line configured to optimize fuel economy is normally selected, but when the available reducing agent in the exhaust gas purification system has decreased beyond a certain level, a different operation line configured to reduce NOx emission from the engine is selected at the expense of fuel economy.
However, when the operation line is changed during the operation of the engine, the characteristics of the engine may change to such an extent that a discomfort may be caused to the vehicle operator.